The present invention is generally related to feeding of webs of any material towards a work station, particularly but not exclusively to a forming press. More particularly, the invention is directed to a feeding device for web material, of the type comprising step-feed means of the web along a longitudinal direction comprising a stationary gripper and a movable gripper, a pneumatic actuator assembly for operating linear alternative forward and back displacement of the movable gripper relative to the stationary gripper parallelly to said longitudinal direction and opening and closure of said grippers in synchronism with the alternative displacement of the movable gripper, and an adjustable stop member cooperating with the movable gripper at the end of the forward displacement thereof. In such feeding devices the pneumatic actuator assembly traditionally comprises a block incorporating a three-way pneumatically piloted control valve, which controls the communication between said pneumatic actuator assembly and, respectively, a source of air under pressure and a discharge port.
Feeding devices of the above mentioned type are currently manufactured and marketed for instance by ELMER of Turin, Italy. In these known feeding devices, the control valve traditionally comprises a hollow body having an inlet axial passage communicating with a feeding line from said pressurised air source, a radial inlet passage communicating with a pneumatic piloting line, a first and a second radial outlet passages respectively communicating with the pneumatic actuator assembly and with the discharge, and a spool having at one end thereof a plunger subject to the pressure from the piloting line, and having at the other end thereof a floating obturator which, in presence of the piloting pressure, face-sealingly closes said inlet axial passage and opens the communication between the first and the second radial outlet passages, and in the absence of the piloting pressure closes the second radial outlet passage and opens the communication between the inlet axial passage and the first radial outlet passage. In other words, in presence of piloting pressure the pneumatic actuator assembly controlled by the valve is connected to the discharge, while in the absence of piloting pressure the pneumatic actuator assembly is connected to the source of air under pressure.
A feeding device for web material according to the prior art, provided with such a control valve, is shown in FIGS. 1 through 4 of the drawings.
In FIG. 1, reference numeral 1 generally designates a feeding device for web material according to the prior art, which is intended to be combined with a support and guide system of a web, known per se and not shown in the drawings.
The feeding device 1 comprises, a block 2 carrying a stationary gripper 3, a movable gripper 4 displaceable along guides 5 according to a linear alternative motion parallelly to a longitudinal feed direction F of the web, and an adjustable stop member 6 cooperating with the movable gripper 4 at the end of the forward displacement thereof.
Displacement of the movable gripper 4 and opening and closure of the stationary gripper 3 and the movable gripper 4 are operated by means of a pneumatic actuator assembly, not shown in the drawings for the sake of brevity, which is incorporated within the block 2 and is in turn controlled by means of an actuating valve device 7, directly operated mechanically, for instance by the forming press to which the feeding device 1 may be in use associated, or by a pneumatic or electropneumatic remote control, in a way known per se.
The pneumatic actuator assembly incorporated in the block 2 includes a three-way control valve 8, named as "secondary valve", controlling the communication between the pneumatic actuator assembly and, respectively, an external source of pressurised air (not shown) and a discharge port.
Referring now in more detail to FIGS. 2 and 4, the control valve 8 according to the prior art comprises a hollow valve body 9 housed within a rear cylindrical seat 10 of the block 2 and defining a first chamber 11 and a second elongated chamber 12, coaxial to each other. The first chamber 12 is connected, at the end thereof opposite to the first chamber 11, with an axial inlet passage 13 formed through the valve body 9 and communicating with a feed line A, formed within the block 2 and connected to the source of air under pressure.
The first chamber 11 is connected, in correspondence of the end thereof opposite to the second chamber 12, with one or more radial inlet passages 14 communicating, through an annular chamber 15 formed in the valve body 9, with a pneumatic piloting line D formed within the block 2, and in turn controlled by means of the control valve 7.
The chamber 12 is further connected, on the side of the chamber 11, with one or more radial passages 16 communicating with an annular chamber 24 formed in the valve body 9 and in turn communicating with a feed line B to the pneumatic actuator group, formed within the body 2. On the side of the axial passage 13, the chamber 12 is connected through a single radial passage 17 and an annular chamber 18 formed in the valve body 9, with a discharge line C also formed within the block 2.
Reference 19 designates a spool which is axially movable within the hollow valve body 9, and which is formed at one end thereof with a plunger 20 sealingly slidable along the first chamber 11, and at the other end with a stem 21 extending coaxially into the second chamber 12. An obturator 22, floating within the chamber 12, is adjacent to the stem 21.
The obturator 22 has a cylindrical shape with a cross section which is smaller than that of the chamber 12, so as to define with the wall of the latter an axial passage 23, as explained below.
Operation of the known control valve 8 is as follows. In the absence of the piloting pressure in the piloting line D, the spool 19 is maintained in the position depicted in FIG. 2, in which it is shifted to the left with reference to the drawing. In this condition, due to the pressure difference between the chamber 12 and the discharge line C, the lateral surface of the obturator 22 is on one side maintained in contact with the area of the wall of the chamber 12 in correspondence of which the discharge passage 17 opens, while on the other side it defines with the wall of the chamber 12 the axial passage 23.
The radial discharge passage 17 is thus closed, while communication between the axial feed passage 13 and the radial outlet passages 16 is open through the axial passage 23, whereby the air under pressure from the feed line A is fed through the line B to the pneumatic actuator assembly, so as to perform the operating functions of the web feeding device 1.
In presence of the piloting pressure in the line D, the spool 19 is instead placed in the position shown in FIG. 3, in which it is shifted on the right with reference to the drawing, whereby the obturator 22 face-sealingly closes the axial feed passage 13, while opening the communication between the radial passages 16 and 17, i.e. between the line B connected to the pneumatic actuator assembly and the discharge line C. This known construction, while being extremely simple from the point of view of manufacturing, is subject to a functional limit related to the possibility of increasing the air rate of flow from the feed line A to the line B connected to the pneumatic actuator group, to the aim of enhancing the operative functionality of the feed device 1. In fact, in order to increase the air rate of flow, it is necessary to reduce the diameter of the obturator 22, so as to increase the size of the passage 23. However, beyond a certain ratio between the diameter of the obturator 22 and the diameter of the chamber 12, the lateral wall of this obturator 22 is no more able to adhere with a sufficient seal onto the corresponding area of the wall of the chamber 12 and, consequently, in the absence of piloting pressure it is not possible to obtain an airtight closure of the radial discharge passage 17.